SiPMs are semiconductor photon sensitive devices made up of an array of very small Geiger-mode avalanche photodiode (APD) cells on a semiconductor substrate such as silicon. An example 10×10 microcell array is shown in FIG. 1 of the accompanying drawings. Each cell is connected to one another to form one larger device with one signal output. The entire device size can be as small as 1×1 mm or much larger.
APD cells vary in dimension from 10 to 100 microns depending on the mask used, and can have a density of up to 3000 microcells/sq. mm. Avalanche diodes can also be made from other semiconductors besides silicon, depending on the properties that are desirable. Silicon detects in the visible and near infrared range, with low multiplication noise (excess noise). Germanium (Ge) detects infrared to 1.7 μm wavelength, but has high multiplication noise. InGaAs (Indium Gallium Arsenide) detects to a maximum wavelength of 1.6 μm, and has less multiplication noise than Ge. InGaAs is generally used for the multiplication region of a heterostructure diode, is compatible with high speed telecommunications using optical fibres, and can reach speeds of greater than Gbit/s. Gallium nitride operates with UV light. HgCdTe (Mercury Cadmium Telluride) operates in the infrared, to a maximum wavelength of about 14 μm, requires cooling to reduce dark currents, and can achieve a very low level of excess noise.
Silicon avalanche diodes can function with breakdown voltages of 20 to 500V, typically. APDs exhibit internal current gain effect of about 100-1000 due to impact ionization, or avalanche effect, when a high reverse bias voltage is applied (approximately 20-200 V in silicon, depending on the doping profile in the junction). Silicon PhotoMultipliers or SiPMs can achieve a gain of 105 to 106 by using Geiger mode APDs which operate with a reverse voltage that is greater than the breakdown voltage, and by maintaining the dark count event rate at a sufficiently low level. The current generated by an avalanche event must be quenched by an appropriate current limited scheme so that the device can recover and reset after an avalanche event.
Many SPM [Silicon Photomultiplier] applications call for a fast light-to-current response, with order of 1 ns or even shorter time constants. Improved time response would benefit such applications as time-resolved spectroscopy, LIDARs, TOF [time of flight] PET [Positron Emission Tomography] etc. One such SPM is described in U.S. Pat. No. 9,029,772 which is assigned to the present Applicant and the contents of which is incorporated herein by reference. U.S. Pat. No. 9,029,772 includes a fast output terminal arrangement which is illustrated as FIG. 3 in the present Application.
The robustness of an IC to electro static discharge (ESD) is an important consideration that is determined by its capability to safely discharge a high current pulse generated during an ESD event without developing excessive voltage levels or heating that can cause damage to devices on the IC.
There is therefore a need to provide a semiconductor photomultiplier which addresses at least some of the drawbacks of the prior art.